Yeast infections come from an overgrowth of a fungus called Candida that already lives on and inside your body. It’s not something you “catch” in most cases. The fungus is a normal part of your skin, mouth, gut, and vaginal microbiome, kept in check by beneficial bacteria and your immune system. When something disrupts that balance, Candida multiplies beyond its usual levels and causes symptoms. An estimated 75% of women will experience at least one vaginal yeast infection in their lifetime, and 40% to 45% will have two or more episodes.
How the Fungus Normally Stays Under Control
Your body maintains an ecosystem of microorganisms that compete for space and resources. In the vagina, the key players are bacteria from the Lactobacillus family. These bacteria break down a sugar called glycogen (produced by the vaginal lining) into lactic acid, which keeps the environment acidic, typically at a pH between 3.8 and 4.5. That acidity makes it difficult for Candida to multiply.
Not all Lactobacillus species are equally protective. Research has shown that women whose vaginal microbiome is dominated by one particular strain, Lactobacillus crispatus, are significantly less likely to harbor Candida than women dominated by a different strain called Lactobacillus iners. The reason: L. crispatus produces more lactic acid and inhibits Candida growth more effectively. This helps explain why two people with seemingly similar habits can have very different susceptibility to yeast infections.
The same general principle applies in your mouth and on your skin. Competing microbes and a functioning immune system keep Candida populations small. When either of those defenses weakens, the fungus takes advantage.
Antibiotics Are the Most Common Trigger
Broad-spectrum antibiotics, the kind prescribed for sinus infections, bronchitis, urinary tract infections, and many other conditions, don’t distinguish between harmful bacteria and helpful ones. They wipe out Lactobacillus along with the bacteria causing your illness. Without enough Lactobacillus, the vagina becomes less acidic, and Candida has room to grow unchecked. This is why many women develop a yeast infection during or shortly after a course of antibiotics.
The longer the antibiotic course and the broader the spectrum, the higher the risk. If you’ve noticed a pattern of yeast infections following antibiotics, it’s not a coincidence. It’s one of the most well-established cause-and-effect relationships in vaginal health.
Hormones Play a Direct Role
Estrogen influences how much glycogen (the sugar that feeds both Lactobacillus and Candida) your vaginal lining produces. When estrogen levels are high, glycogen production increases. Under normal circumstances, Lactobacillus uses that glycogen to produce lactic acid and maintain acidity. But when estrogen surges are dramatic or when other disruptions are happening at the same time, the extra glycogen can also fuel Candida growth.
This is why yeast infections are more common during pregnancy, in the second half of the menstrual cycle, and among people using hormonal contraceptives with higher estrogen doses. On the flip side, after menopause, estrogen drops and glycogen production falls. This reduces both Lactobacillus and Candida populations, which is why postmenopausal women tend to get fewer yeast infections (though they become more vulnerable to other types of vaginal irritation).
High Blood Sugar Feeds the Fungus
Diabetes is one of the strongest medical risk factors for yeast infections, and the mechanism is straightforward. When blood sugar runs high, excess sugar shows up in vaginal secretions, urine, and saliva, all of which create a richer food source for Candida. According to the CDC, women with diabetes have a higher risk of vaginal yeast infections, particularly when their blood sugar isn’t well controlled.
This connection goes beyond diagnosed diabetes. Periods of consistently elevated blood sugar from any cause, including prediabetes or a diet very high in refined carbohydrates, can shift the balance in Candida’s favor. If you’re getting frequent yeast infections and don’t know why, it’s worth having your blood sugar checked.
A Weakened Immune System
Your immune system is the other major line of defense against Candida overgrowth. Conditions that suppress immune function, including HIV/AIDS, cancer treatments like chemotherapy, organ transplant medications, and long-term use of corticosteroids (including inhaled steroids for asthma), all raise the risk of yeast infections throughout the body. People with severely compromised immunity can develop systemic Candida infections that spread beyond the skin and mucous membranes, which is a more serious medical concern.
Even temporary immune dips from illness, poor sleep, or high stress can sometimes be enough to tip the balance, though these are harder to quantify than the major medical risk factors.
Clothing, Moisture, and Everyday Habits
Candida thrives in warm, moist environments. Anything that traps heat and moisture against your skin gives the fungus better growing conditions. Cleveland Clinic recommends 100% cotton underwear because it wicks away excess sweat and moisture that yeast thrives on. Synthetic fabrics, even those with a cotton crotch panel, don’t breathe as well and don’t offer the same protection.
Other practical factors that increase moisture and warmth in the genital area include tight-fitting clothing, sitting in a wet swimsuit for hours, and wearing panty liners daily when they aren’t needed. Going without underwear at night, especially during an active infection, increases airflow and can help with healing. Loose pajamas or boxer shorts serve the same purpose.
These habits alone rarely cause a yeast infection in someone with a well-balanced microbiome. But for people who are already prone to them, reducing moisture can meaningfully lower the frequency of recurrences.
Yeast Infections Beyond the Vagina
Vaginal yeast infections get the most attention, but Candida overgrowth can happen in several other places on the body.
- Oral thrush appears as white patches on the tongue and inner cheeks. The risk factors overlap heavily with vaginal yeast infections: antibiotics, diabetes, weakened immunity, and corticosteroid use (particularly inhaled steroids that contact the mouth and throat). Wearing dentures and having chronic dry mouth also increase the risk.
- Penile yeast infections cause redness, irritation, and sometimes a rash on the head of the penis. Men are more likely to develop them if they’re uncircumcised, have diabetes, use antibiotics long-term, have a weakened immune system, or are overweight. Sexual contact with a partner who has a vaginal yeast infection can also transmit Candida, though this isn’t the most common route.
- Skin fold infections develop in areas where skin touches skin and traps moisture: under the breasts, in the groin, between fingers, and in the armpits. Excess weight increases the surface area of these folds and the amount of trapped moisture.
In all these locations, the underlying cause is the same: something disrupted the local environment enough to let Candida outcompete its neighbors.
Why Some People Get Recurrent Infections
Fewer than 5% of women experience recurrent yeast infections, defined as three or more episodes in a single year. For these individuals, the cause is often a combination of factors rather than one clear trigger. Their vaginal microbiome may naturally lean toward less protective bacterial strains, their immune response to Candida may be less aggressive, or they may have an ongoing condition like uncontrolled diabetes that continuously creates favorable conditions for the fungus.
Recurrent infections can also happen when a previous infection wasn’t fully cleared. Some Candida strains are more resistant to standard antifungal treatments, which means the fungus gets knocked back but never eliminated, then rebounds once treatment stops. If over-the-counter treatments aren’t keeping infections away, testing to identify the specific Candida species involved can help guide more effective treatment.